The present invention relates in general to plastic metal working and more specifically to constructions of machinery, for instance mills for rolling axially symmetric parts such as wheels and disks, and can find application for producing such parts from low-plastic hard-to-work materials, such as superalloys.
A number of prior-art mills for producing axially symmetric parts are known to comprise a working tool usually appearing as a pair of rotatable rolls fitted in rolling heads movable with respect to the billet under process by means of carriages which in turn are traversable along bedways. The billet is fitted in the mill rotatably about its own axis (cf. USSR Inventor's Certificate No. 275,039).
The aforementioned devices are used for producing such parts as the wheels of railway stock which are comparatively simple in construction, from materials that are plastic within a wide temperature range, such as carbon steels.
One more prior-art device is known to comprise a top and a bottom roll for shaping the wall of a part, both rolls being driven in rotation in a definite sense and arranged opposite to each other on both sides of the walls of a disk-like billet with a possibility of synchronous up-and-down and radial motion imparted by appropriate drives, a rotatable mandrel supported on a stationary fixed vertical axle, a rotatable and vertically movable side roll arranged opposite to the rotatable mandrel and aimed at supporting the billet between said roll and the mandrel, a top and a bottom edging roll exerting vertical pressure on the billet surface and supported with a possibility of radial motion, radially movable guide rollers rotatable while getting in contact with the billet end face, and a number of rotatable backup rollers (cf. Japanese Patent Publication SHO-61-11696).
The device mentioned before is suitable, due to a complicated kinematic features of the tool and provision of numerous rolls, for producing disk-type parts having more intricate configuration likewise from plastic materials.
The above-described devices cannot, however, be used for producing such parts as gas-turbine disks from hard-to-work high-temperature materials, e.g., nickel- and titanium-base alloys.
A further prior-art rolling mill is known to comprise bed-mounted rolling heads whose number is multiple of two, provided with drives imparting rotation, vertical and horizontal motion thereto, each of said rolling heads having its own carriage which is bed-mounted and independently movable in mutually square direction in a horizontal plane, and a mechanism for turning the rolling head in a horizontal plane. The mill under consideration further comprises a working furnace into which the billet is placed after having been preheated in a preheater furnace. The mill further comprises two coaxial billet holding units and a mechanical actuator imparting rotation thereto. The working furnace has openings for the rolls and part of the billet holding unit to insert therein. The billet holding unit comprises also a mandrel. The required contour of the part being produced is formed when the rolls move from the center towards the periphery along a preset pathway (cf. RU Patent No. 2,031,753).
The mill, according to said patent, is suitable for producing a number of parts, predominantly simple in shape, from hard-to-work alloys. With a view to observing isothermal or superplastic conditions, plastic working of such billets should be performed in a working furnace. This is necessary for increasing the plasticity of material and reducing its yield stress. At high temperatures the material features a low yield stress. Uncontrolled additional strain of the already worked billet areas occurs due to too a low yield stress, as well as owing to the fact that under superplastic conditions the dimensions of the strain center exceed substantially the area of direct action of the rolls on the billet under process. This in turn results ultimately in spoilage. Use of a mandrel to some extent restricts unintentional thinning of the billet; however, a mandrel can be used only in producing parts of a relatively small diameter having but a small difference in thickness between the already rolled billet areas and its area under rolling. Otherwise the plastically worked billet areas get thinned despite the use of a mandrel, this being due to the heavy rolling forces to be applied. That is why the mill in question is applicable only for producing disk-type axially symmetric parts from hard-to-work alloys, having relatively simple configuration and a diameter of from 500 to 800 mm, with large preset working allowances for fear of thinning the web during the rolling process.
It is noteworthy that up-to-date requirements imposed on critical parts produced by plastic working techniques include not only attaining an accurate shape and size of a part approximating the finished ones but also its microstructure, which is to a great extent decisive for mechanical and performance characteristics of the part involved. The preset specified microstructure of such parts is to be established during their plastic working, for which purpose a device for their producing must provide varying thermal and mechanical conditions of the working process. However, such a possibility is not provided in the known devices of the character set forth before.